(i) Field of the Invention
The present invention relates to active agent delivery systems and, more specifically, to compositions that comprise peptides covalently attached to active agents and methods for administering conjugated active agent compositions.
(ii) Background of the Invention
Active agent delivery systems are often critical for the safe effective administration of a biologically active agent (active agent). Perhaps the importance of these systems is best realized when patient compliance and consistent dosing are taken under consideration. For instance, reducing the dosing requirement for a drug from four-times-a-day (QID) to a single dose per day would have significant value in terms of ensuring patient compliance. Increasing the stability of the active agent, will assure dosage reproducibility and perhaps may also reduce the number of dosages required. Furthermore, any benefit that modulated absorption can confer on an existing drug would certainly improve the safety of the drug. Finally, improving the absorption of drugs should have a significant impact on the safety and efficacy of the drug.
Absorption of an orally administered active agent is often blocked by the harshly acidic stomach milieu, powerful digestive enzymes in the gastrointestinal (GI) tract, lack of the agent's permeability and transport across lipid bilayers. These systems respond, in part, to the physicochemical properties of the drug molecule itself. Physical constants describing specific physicochemical properties like lipophilicity (log P) and ionizability (pKa) depend on molecular structure of the active agent. Some drugs are poorly absorbed because they are too hydrophilic and do not effectively cross the plasma membranes of cells. Others are too lipophilic and are insoluble in the intestinal lumen and cannot migrate to the mucosa lining. The entire digestion and absorption process is a complex sequence of events, some of which are closely interdependent. There should exist optimum physicochemical properties by which active agent bioavailability is maximized. However, it is often difficult to optimize these properties without losing therapeutic efficacy.
Optimization of a drug's bioavailability has many potential benefits. For patient convenience and enhanced compliance it is generally recognized that less frequent dosing is desirable. By extending the period through which the drug is absorbed, a longer duration of action per dose is expected. This will then lead to an overall improvement of dosing parameters such as taking a drug twice a day where it has previously required four times a day dosing. Many drugs are presently given at a once a day dosing frequency. Yet, not all of these drugs have pharmacokinetic properties that are suitable for dosing intervals of exactly twenty-four hours. Extending the period through which these drugs are absorbed would also be beneficial.
One of the fundamental considerations in drug therapy involves the relationship between blood levels and therapeutic activity. For most drugs, it is of primary importance that serum levels remain between a minimally effective concentration and a potentially toxic level. In pharmacokinetic terms, the peaks and troughs of a drug's blood levels ideally fit well within the therapeutic window of serum concentrations. For certain therapeutic agents, this window is so narrow that dosage formulation becomes critical. Such is the case with the drug, digoxin, which is used to treat heart failure.
Digoxin therapeutic blood levels include the range between 0.8 ng/mL (below which the desired effects may not be observed) and about 2 ng/mL (above which toxicity may occur). Among adults in whom clinical toxicity has been observed, two thirds have serum concentrations greater than 2 ng/mL. Furthermore, adverse reactions can increase dramatically with small increases above this maximum level. For example, digoxin-induced arrhythmias occur at 10%, 50%, and 90% incidences at serum drug levels of 1.7, 2.5 and 3.3 ng/mL, respectively.
After the oral administration of digoxin, an effect will usually be evident in 1-2 hours with peak effects being observed between 4 and 6 hours. After a sufficient time, the concentration in plasma and the total body store is dependent on the single daily maintenance dose. It is critical that this dose be individualized for each patient. Having a dosage form of digoxin that provides a more consistent serum level between doses is therefore useful.
Another example of the benefit of more consistent dosing is provided by the β-blocker atenolol. The duration of effects for this commonly used drug is usually assumed to be 24 hours. However, at the normal dose range of 25-100 mg given once a day, the effect may wear off hours before the next dose begins acting. For patients being treated for angina, hypertension, or for the prevention of a heart attack, this may be particularly risky. One alternative is to give a larger dose than is necessary in order to get the desired level of action when the serum levels are the lowest. But this may cause side effects related to excessive concentrations in the initial hours of the dosing interval. At these higher levels, atenolol loses its potential advantages. β-1 selectivity and adverse reactions related to the blockade of β-2 receptors become more significant.
In the case of anti-HIV nucleoside drugs, metering the absorption of the drug into the bloodstream has sufficient benefit. Drugs like AZT, for example, depend on phosphorylation to occur after absorption and before uptake into a virus in order to be effective. In normal dosing, drug levels increase rapidly after absorption that the phosphorylation reaction pathways can become saturated. Furthermore, the rate of phosphorylation is dependent on serum concentrations. The reactions occur more rapidly when concentrations are lower. Therefore, nucleoside analogs which retain lower serum concentrations are more efficiently converted to active drugs than other rapidly absorbed anti-viral drugs.
Whereas the toxicity of digoxin and atenolol can be viewed as extensions of their desired activities, toxicity associated with the statins, on the other hand, is seemingly unrelated to the therapeutic effect of the drug. The toxic side effects of statins include, amongst other things, liver problems and rhabdomyolysis. Although the exact cause of statin-induced rhabdomyolysis and liver toxicity is not well understood, they have been linked to potent liver enzymes. The therapeutic effect of the statins is a result of the down-regulation of one of the key enzymes responsible for cholesterol production. Statin overdosing, however, can cause the reduced synthesis of non-sterol products that are important for cell growth, in addition to rhabdomyolysis. So with the statins, a case can be made that by modulating the absorption of the drug, the therapeutic effect can be obtained at lower doses thereby minimizing the risk of producing toxic side effects.
Finally, increasing the absorption of an active agent can have a significant impact on its safety. Taking the example of statins, once more, statins are anywhere between 10 and 30% absorbed and dosing is based on the average of this range so for patients that absorb 30% of the statins administered, deleterious side effects can occur. The risk of manifesting these side effects would be greatly diminished if the bioavailability of the drug were more predictable.
In an attempt to address the need for improved bioavailability several drug release modulation technologies have been developed. Enteric coatings have been used as a protector of pharmaceuticals in the stomach and microencapsulating active agents using protenoid microspheres, liposomes or polysaccharides have been effective in abating enzyme degradation of the active agent. Enzyme inhibiting adjuvants have also been used to prevent enzyme degradation.
A wide range of pharmaceutical formulations purportedly provides sustained release through microencapsulation of the active agent in amides of dicarboxylic acids, modified amino acids or thermally condensed amino acids. Slow release rendering additives can also be intermixed with a large array of active agents in tablet formulations. For example, formulating diazepam with a copolymer of glutamic acid and aspartic acid enables a sustained release of the active agent. As another example, copolymers of lactic acid and glutaric acid are used to provide timed release of human growth hormone. The microencapsulation of therapeutics and diagnostic agents is generally described for example in U.S. Pat. No. 5,238,714 to Wallace et al.
While microencapsulation and enteric coating technologies impart enhanced stability and time-release properties to active agent substances these technologies suffer from several shortcomings. Incorporation of the active agent is often dependent on diffusion into the microencapsulating matrix, which may not be quantitative and may complicate dosage reproducibility. In addition, encapsulated drugs rely on diffusion out of the matrix or degradation of the matrix, which is highly dependant on the water solubility of the active agent. Conversely, water-soluble microspheres swell by an infinite degree and, unfortunately, may release the active agent in bursts with little active agent available for sustained release. Furthermore, in some technologies, control of the degradation process required for active agent release is unreliable. For example, an enterically coated active agent depends on pH to release the active agent and, as such, is difficult to control the rate of release.
Active agents have been covalently attached to the amino acid side chains of polypeptides as pendant groups. These technologies typically require the use of spacer groups between the amino acid pendant group and the active agent. An example of a timed and targeted release pharmaceutical administered intravenously, intraperitoneally, or intra-arterially includes the linking of nitrogen mustard, via a stabilizing peptide spacer, to a macromolecular carrier, e.g. poly-[N5-(2-hydroxylethyl)-L-glutamine] (PHEG) which has an improved half-life when attached to the carrier and stabilizing unit.
Dexamethasone has been covalently attached directly to the beta carboxylate of polyaspartic acid as a colon-specific drug delivery system, which is released by bacterial hydrolytic enzymes residing in the large intestines. The dexamethasone active agent was targeted to treat large bowel disorders and was not intended to be absorbed into the bloodstream. Other examples include techniques for forming peptide-linked biotin, peptide-linked acridine, naphthylacetic acid bonds to LH—RH, and coumarinic acid cyclic bonds to opioid peptides.
Several implantable drug delivery systems have utilized polypeptide attachment to drugs. An example includes the linking of norethindrone, via a hydroxypropyl spacer, to the gamma carboxylate of a large polyglutamic acid polymeric carrier designed to biodegrade over long periods of time after implantation via injection or surgical procedures. Additionally, other large polymeric carriers incorporating drugs into their matrices are used as implants for the gradual release of drug. Examples of implant delivery systems are generally described in U.S. Pat. No. 4,356,166 to Peterson et al. and U.S. Pat. No. 4,976,962 to Bichon et al. Yet another technology combines the advantages of covalent drug attachment with liposome formation where the active ingredient is attached to highly ordered lipid films (known as HARs) via a peptide linker. Further description can be found in WO 97/36616 and U.S. Pat. No. 5,851,536 to Yager et al.
Other systems were designed for the delivery of cytotoxic agents with specific amino acid sequences so that the drug will not be cleaved until the conjugate comes into contact with specific enzymes or receptors. One such example is the binding of oligopeptides directed toward enzymatically active prostrate specific antigen (PSA), to a cytotoxic agent which also typically contains a protecting group to prevent premature release of the conjugate in the blood. Another system designed for delivery via injection relies on specific peptides directly linked to a polymeric carrier, which is, in turn, attached to the cytotoxic moiety to be internalized by the cell. In this case, the polymeric carrier, typically large, will not enter cells lacking receptors for the specific peptide attached. In another example, gastrin receptor directed tetragastrin and pentagastrin amides were attached to cytotoxic drug for testing in vitro. These systems are generally described in U.S. Pat. No. 5,948,750 to Garsky et al.; U.S. Pat. No. 5,087,616 Myers et al.; and Schmidt et al., Peptide Linked 1,3-Dialkyl-3-acyltriazenes: Garstrin Receptor Directed Antineoplastic Alkylating Agents, Journal of Medical Chemistry, Vol. 37, No. 22, pp. 3812-17 (1994).
Several systems have been directed to the treatment of tumor cells. In one case, Daunorubicin bound poly-L-aspartic acid, delivered intravenously or intraperitoneally, demonstrated lower cytotoxic effect. In another example, Daunorubicin was covalently attached via a methylketone side-chain of the drug to both poly-L-aspartic acid and poly-L-lysine The conjugates were prepared for intravenous and intraperitoneal administration. The poly-L-lysine was found to be ineffective, while the poly-L-aspartic acid conjugate showed preferential tumor cell uptake. In another system, a highly substituted polypetide conjugated to the active agent was designed for introduction into blood vessels that further penetrated the interstitium of tumors through long chain lengths. Further discussion of delivery systems targeted at tumor cells are described in Zunino et al., Comparison of Antitumor Effects of Daunorubicin Covalently Linked to Poly-L-Amino Acid Carriers, Eur. J. Cancer Clin. Oncol., Vol. 20, No. 3, pp. 421-425 (1984); Zunino et al., Anti-tumor Activity of Daunorubicin Linked to Poly-L-Aspartic Acid, Int. J. Cancer, 465-470 (1982); and U.S. Pat. No. 5,762,909 to Uzgiris.
Several examples relate to the delivery of paclitaxel, an anti-cancer drug. One delivery system relies on the drug remaining attached to the conjugate for transport into the cell via a membrane transport system. In another example, the paclitaxel is conjugated to a high molecular weight polyglutamic acid, and was delivered via injection. Paclitaxel conjugates have also been used with implants, coatings and injectables. These systems are further described in U.S. Pat. No. 6,306,993; Li et al. Complete Regression of Well-established Tumors Using a Novel Water-soluble Poly(L-Glutamic Acid)-Paclitaxel Conjugate, pp. 2404-2409; and U.S. Pat. No. 5,977,163 to Li et al.
Delivery systems can be designed to utilize attachment to chemical moieties that are either specifically recognized by a specialized transporters or have enhanced adsorption into target cells through specific polypeptide sequence. There are seven known intestinal transport systems classified according to the physical properties of the transported substrate. They include the amino acid, oligopeptide, glucose, monocarboxic acid, phosphate, bile acid and the P-glycoprotein transport systems and each has its own associated mechanism of transport. Evidence suggests that hydrophilic compounds are absorbed through the intestinal epithelia more efficiently through these specialized transporters than by passive diffusion. Active transport mechanisms can depend on hydrogen ions, sodium ions, binding sites or other cofactors. Facilitation of these transporters can also depend on some sort of specialized adjuvant, which can result in localized delivery of an active agent, increased absorption of the active agent or some combination of both. Incorporating adjuvants such as resorcinol, surfactants, polyethylene glycol (PEG) or bile acids enhance permeability of cellular membranes. Increased bioavailability was found when peptides were bound to modified bile acids. Kramer et al., Intestinal Absorption of Peptides by Coupling to Bile Acids, The Journal of Biological Chemistry, Vol. 269, No. 14, pp. 10621-627 (1994). The use of specific polypeptide sequences to increase absorption is discussed in the literature where attaching the drug to a polypeptide chain enhances the drug's permeability into cells. For example, Paul Wender of Stanford University reported the use of polyarginine tags on cyclosporine and taxol to facilitate diffusion across cell membranes. The penetratin system provides another example. This class of peptides, about 16 residues long, was shown to enhance absorption of oligonucleotides and polypeptides.
It is also important to control the molecular weight, molecular size and particle size of the active agent delivery system. Variable molecular weights have unpredictable diffusion rates and pharmacokinetics. High molecular weight carriers are digested slowly or late, as in the case of naproxen-linked dextran, which is digested almost exclusively in the colon by bacterial enzymes. High molecular weight microspheres usually have high moisture content which may present a problem with water labile active ingredients. Due to the inherent instability of non-covalent bonds, the bond between the active agent and the microsphere will usually not withstand the vigorous conditions used to reduce the composition's particle size.
Thus, there has been no pharmaceutical composition, heretofore reported, that incorporates the concept of attaching an active agent to a peptide or its pendant group with targeted delivery of the active agent into the general systemic circulation. Furthermore, there has been no pharmaceutical composition that teaches the release of the active agent from the peptide by enzymatic action in the gastro intestinal tract.
The need still exists for an active agent delivery system that does not require that the active agent be released within specific cells (e.g. a tumor-promoting cell), but rather results in release of the active agent for general systemic delivery.
The need further exists for an active agent delivery system that allows for the oral delivery of active agents that will survive in the stomach and allow for the release of the reference active agent in the small intestines, the brush border membrane, in the intestinal epithelial cells or by enzymes in the bloodstream. The present invention also addresses the need for an active agent delivery system that is able to deliver active agents as an active agent peptide conjugate so that the molecular mass and physiochemical properties of the conjugate can be readily manipulated to achieve the desired release rate. The need still exists for an active agent delivery system that allows for the active agent to be released over a sustained period of time, which is convenient for patient dosing.
There is a generally recognized need to have an active agent delivery system that reduces the daily dosing requirement and allows for time released or controlled released absorption of the composition. The present invention accomplishes this by extending the period during which an active agent is absorbed, and providing a longer duration of action per dose than is currently expected. This leads to an overall improvement of dosing parameters such as, for example, taking an active agent twice a day where it has previously required four times a day dosing. Alternatively, many active agents presently given at a once a day dosing frequency lack the pharmacokinetic properties suitable for dosing intervals of exactly, twelve or twenty-four hours. An extended period of active agent adsorption for the current single dose active agents still exists and would be beneficial.
Therefore, the need still exists for a drug delivery system that enables the use of new molecular compositions, which can reduce the technical, regulatory, and financial risks associated with active agents while improving the performance of the active agent in terms of its absorption parameters, as described above, and stability. Further, the need exists for an active agent delivery system targeted to general systemic circulation wherein the release of the drug from the peptide can occur by enzymatic action on the peptide-drug conjugate in the bloodstream or by enzymatic action on the peptide-drug conjugate in the alimentary tract followed by absorption through the intestines or stomach.